Wireless terminals are widely used for voice and/or data communications. As used herein, the term “wireless terminal” encompasses a wide variety of portable or fixed wireless devices that can access a wireless communications system. Wireless terminals include a cellular radiotelephone with a multi-line display, a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with data processing, facsimile and/or data communications capabilities, a Personal Digital Assistant (PDA) that can include a radiotelephone, pager, Internet/intranet access, Web browser, organizer, calendar and/or a Global Positioning System (GPS) receiver, and conventional laptop, palmtop and/or pervasive computing devices that include wireless receivers.
It is often desirable for a wireless terminal to transmit and receive over multiple frequency bands, for example to provide both PCS and GSM capabilities. It also may be desirable to provide wireless terminals that operate over four RF bands. For example, it may be desirable to provide a wireless terminal that can operate over the GSM850 band that is used in the United States (also referred to herein simply as GSM), the EGSM900 band that is used in Europe (also referred to herein simply as EGSM), the DCS1800 band that is used in Europe (also referred to herein simply as DCS) and the PCS1900 band that is used in the United States (also referred to herein simply as PCS). The transmit (TX) and receive (RX) frequencies of these bands are shown in Table 1:
TABLE 1Frequency RangeBand 824 Mhz–849 MhzGSM850 TX Band: US 869 Mhz–894 MhzGSM850 RX Band: US 880 Mhz–915 MhzEGSM900 TX Band: Europe 925 Mhz–960 MhzEGSM900 RX Band: Europe1710 Mhz–1785 MhzDCS1800 TX Band: Europe1805 Mhz–1880 MhzDCS1800 RX Band: Europe1850 Mhz–1910 MhzPCS1900 TX Band: US1930 Mhz–1990 MhzPCS1900 RX Band: US
A quad band antenna interface module may be used to interface between an antenna port and the transmit and receive ports for the four bands. Unfortunately, it may be difficult to design a quad band antenna interface module that can provide satisfactory RF performance at low cost. In particular, it may be difficult to obtain a satisfactory Voltage Standing Wave Ratio (VSWR) and/or to obtain satisfactory port-to-port isolation in a quad band antenna interface module.
As is well known to those having skill in the art, VSWR is a figure of merit that may be used to quantify the port mismatch loss between an energy source and a load. The relationship between VSWR and source-to-load mismatch loss is generally defined by the following equation:
                    Mismatch_Loss        =                  10          *                      Log            ⁡                          (                              1                -                                                      [                                                                  VSWR                        -                        1                                                                    VSWR                        +                        1                                                              ]                                    2                                            )                                                          (        1        )            A 2:1 VSWR therefore translates into a 0.5 dB loss. A 3:1 VSWR translates into a 1.25 dB loss. A 4:1 VSWR translates into a 1.93 dB loss. This extra insertion loss may directly impact the overall RF performance of the wireless terminal. Since the remainder of the wireless terminal performance may be fixed, it may be desirable to reduce or minimize the port mismatch loss so as to achieve satisfactory wireless terminal performance.
Moreover, wireless terminals may increasingly use internal antenna structures. Compared to external antenna structures, internal antennas may present a much larger VSWR range to the RF circuitry. The use of internal antennas may make it even more difficult to reduce or minimize mismatch loss over the four bands.
As noted above, it may also be difficult to obtain a desirable transmit port-to-receive port isolation. In particular, it is known that transmit energy feedback into the wireless terminal circuitry may cause voltage controlled oscillator frequency pulling, which may cause transmit mode frequency error. Unfortunately, however, it may be difficult to obtain sufficient TX-to-RX isolation in the above-described quad band frequencies as a result of frequency band overlaps.
In particular, FIG. 1 graphically illustrates frequency overlaps in the quad band frequencies of Table 1. Arrows 110, 120 and 130 represent frequency band overlap regions between the EGSM transmit frequency band and the GSM receive frequency band, between twice the EGSM transmit frequency band and the DCS1800 receive band, and between the PCS transmit band and the DCS receive band, respectively. These overlap regions may define the worst case attenuation needs. The overlap region represented by arrow 130 may represent the toughest isolation requirement. It may be desirable to provide about 35 dB or more of PCS TX to DCS RX isolation to reduce or minimize transmitter modulation errors.
Quad band antenna interface modules have been proposed. For example, a quad band interface module marketed by muRata under the designation LMSP-0032TEMP is illustrated in FIG. 2 and is described, for example, in the Specification Goal No. SP-LMZ-88, entitled Switchplexer™ for QuadBand, Tentative Part Number LMSP-0032TEMP (2002). Integrated receive Surface Acoustic Wave (SAW) filters may be available for this module in 2003.
A quad band antenna interface marketed by Panasonic under the designation GNO40xxN is illustrated in FIG. 3, and is described in the Preliminary Specification entitled GN040xxN: SP6T GaAs Antenna Switch Module, 9 Jan. 2002. A quad band interface module marketed by Sony under the designation CXGSP6TM has a similar architecture. Both of these modules may integrate RX SAW filters in 2003.
Unfortunately, the above-described quad band antenna interface modules may not provide sufficient impedance matching between the switching core thereof and the receiver SAW filters, which may result in lower worst case receiver sensitivity and/or degraded audio. Moreover, the DCS/PCS_TX port to DCS_RX port isolation may be insufficient, which may translate into TX mode peak phase error and/or TX link modulation distortion. The EGSM_TX port to DCS_RX port isolation at twice the transmit frequency also may be insufficient, which may translate into TX mode peak phase error and/or TX link modulation distortion. Finally, the EGSM_TX port to GSM850_RX port isolation at twice the transmit frequency may be insufficient and may translate into TX mode peak phase error and/or TX link modulation distortion.